The present invention relates to a drum brake apparatus in which a highly effective and stable braking can be secured by controlling a pressing force of a brake shoe against a drum according to a brake power. More specifically, an improvement for realizing downsizing of operating force generating means or improvement of responsiveness of a braking action by reducing displacement in output from the operating force generating means.
In the related art, various types of drum brake apparatuses in various forms are employed for braking the traveling of a vehicle. These drum brake apparatuses are classified into leading-trailing type, two-leading type, or a duo-servo type depending on the arrangement of brake shoes to be pressed against the inner peripheral surface of a substantially cylindrical drum.
The duo-servo type drum brake apparatus is generally provided with a pair of brake shoes including a primary shoe and a secondary shoe disposed in a cylindrical drum so as to oppose to each other.
The primary shoe has a power input section on the entrance side of the drum in the direction of forward rotation, and the exit side of the drum in the direction of forward rotation is connected to the entrance side of the secondary shoe, for example, via an adjuster. On the other hand, the exit side of the secondary shoe is brought into abutment with an anchor unit provided on a backing plate so that a brake power (braking torque) applied on the primary shoe and the secondary shoe is received by the anchor unit.
Accordingly, when the primary shoe and the secondary shoe are opened outward and pressed against the inner peripheral surface of the drum, the brake power acting on the primary shoe is transmitted to the entrance side of the secondary shoe and acts to press the secondary shoe against the inner peripheral surface of the drum. Therefore, a self-servo effect is generated both on the primary shoe and the secondary shoe, so that a braking force with a fairly high gain is obtained.
The duo-servo type drum brake apparatus described above has a number of advantages in comparison with the drum brake apparatuses of leading-trailing type or two-leading type, not only in that a fairly high braking force can be obtained, but also in that it can easily be downsized and a parking brake can easily be assembled.
However, since the duo-servo type drum brake apparatus as described above is sensitive to variation in coefficient of friction of a lining of the brake shoe, there is a tendency that the braking force can hardly be stabilized, and hence there is a requirement for a device to stabilize the braking force.
In addition, electrification of the brake apparatus is also an important subject for the brake apparatus for a vehicle in these days in order to adapt it to intelligent braking function including an anti-lock braking system or to an electric vehicle (EV) suitable for reducing environmental contamination.
In view of such backgrounds, as a shoe drive mechanism for opening a pair of brake shoes outward during braking period, the present applicant has already proposed a cam mechanism for opening the pair of brake shoes outward and presses the same against the drum depending on a shoe operating force transmitted to an input power receiving portion from operating force generating means when putting a service brake on, and allowing a brake limiting force acting in the direction of reducing the effect of the shoe operating force when the brake power acting on an anchor pin during braking period reaches a predetermined magnification with respect to the shoe operating force (For example, JP-A-2002-333044).
By using the cam mechanism of JP-A-2002-333044, the braking force in the duo-servo type drum brake apparatus was stabilized and, in addition, electrification of the brake apparatus could easily be realized only by employing the electric operating force generating means utilizing an electric motor as an operating force generating means instead of a liquid pressure wheel cylinder in the related art.
However, the cam mechanism as the shoe drive mechanism in the related art as described above includes an input pin for receiving the shoe operating force from the operating force generating means, and a primary anchor pin and a secondary anchor pin which come into abutment with the ends of the respective brake shoes are provided on a cam plate, which is rotatably provided on a main anchor pin fixed to the backing plate, so that the brake shoes are opened outward by the rotation of the cam plate about the main anchor pin according to the shoe operating force. Therefore, the distance from the input pin to the main anchor pin is relatively large in comparison with the distance from the main anchor pin to the primary anchor pin or the distance from the main anchor pin to the secondary anchor pin.
In order to set a lever ratio on the cam plate, displacement transmitted to the cam plate by the operating force generating means is reduced and transmitted to the respective brake shoes, and hence the operating force generating means is required to be capable of outputting a large displacement. Therefore, when a liquid-pressure piston or the like is used as the operating force generating means, there is a possibility to cause problems such as upsizing of the operating force generating means or lowering of the responsiveness of the braking action due to increase in the amount of used liquid in the initial stage of braking.